Reference signals, sometimes also called pilot signals, are widely used in modern communication systems. For example, in communication systems conforming to the Long Term Evolution (LTE) standard of the 3rd Generation Partnership Project (3GPP) reference signals are employed, inter alia, for channel estimation purposes and uplink channel quality signalling.
In the LTE uplink from a user terminal to a base station (in the LTE terminology also denoted as User Equipment, or UE, and evolved Node B, or eNodeB, respectively), so called Demodulation Reference Signals (DMRSs) are transmitted. In the absence of uplink data, the user terminal transmits the DMRS in the Physical Uplink Control Channel (PUCCH). Otherwise, the DMRSs are multiplexed with the uplink data and transmitted together with the uplink data in the Physical Uplink Shared Channel (PUSCH). The base station uses the DMRSs in a channel estimation process that is followed by coherent detection and coherent demodulation as generally known in the art.
Reference signals such as DMRSs are typically generated from sequences of individual sequence elements. Possible sequences have to fulfil certain criteria to be suitable for reference signal generation. For example, to permit efficient channel estimation, the sequences should have a flat frequency domain representation. Moreover, the sequences should have favourable auto- and cross-correlation properties. Specifically, a sequence with a periodic auto-correlation function that is zero in case the sequence is correlated with a cyclically shifted version thereof (and non-zero in case of a zero shift) would be desirable. The latter means that cyclically shifted versions of the sequence are orthogonal to each other, so that multiple orthogonal sequences can be derived from a single base sequence.
One set of base sequences that satisfies these (and further) criteria in an adequate manner are so-called Zadoff-Chu sequences. Zadoff-Chu sequences are used as base sequences for reference signal generation in LTE communication systems (see section 5.5.1.1 of 3GPP Technical Specification (TS) 136.211 V8.9.0 (2010 January) LTE; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation).
The actual reference signal sequence assigned by a base station to a specific user terminal is defined in the LTE standard by a specific cyclic shift of a given Zadoff-Chu base sequence (see section 5.5.1 of 3GPP TS 136.211). The fact that multiple orthogonal sequences are obtainable by impressing different shifts on a given base sequence is exploited in the LTE uplink to reduce the interference between the resulting reference signals.
A further possibility to combat interference introduced with LTE Release 10 are Orthogonal Cover Codes (OCCs). The OCC approach is based on orthogonal time domain codes and operates on the two reference signals transmitted for each LTE sub-frame in the uplink. By means of the OCC code [1 −1], an interfering reference signal can be suppressed as long as its contribution (after a matched filter of the base station) is identical on both reference signals of the same sub-frame. Similarly, the OCC code [1 1] is able to suppress an interfering reference signal as long as its contribution has opposite signs with respect to the two reference signals of the same sub-frame.
In the LTE standard, a specific DMRS is defined by four parameters. In addition to a cyclic shift parameter, a sequence group parameter and a sequence-within-group parameter are defined that jointly define a specific base sequence. Moreover, a sequence length parameter is defined to provide support for different transmission bandwidths in the uplink resource allocation. Different cyclic shifts of a given base sequence are assigned to different user terminals within a cell, whereas different sequence groups (and thus different base sequences) are used in neighbouring cells. While base sequences are assigned in a semi-static manner, cyclic shifts and OCCs are each mobile terminal-specific and dynamically assigned as part of the scheduling grant for each PUSCH transmission.
Simultaneous uplink resource allocations on neighbouring cells can have different transmission bandwidths and can be only partially overlapping in frequency. This fact prevents efficient optimization of reference signal cross-correlation between cells because orthogonality among the reference signals is lost when the transmission bandwidths of the reference signals differ. Orthogonality is also lost when the reference signals are generated from different base sequences (in such a case only “semi-orthogonality” can be achieved). As is readily apparent, a loss of orthogonality among reference signals increases the mutual interference.
To “randomize” inter-cell interference for reference signals, multiple hopping approaches have been defined in the LTE standard. The (pseudo-random) hopping patterns are cell specific and derived from the physical layer cell identity. For PUSCH and PUCCH, LTE supports cyclic shift hopping, sequence group hopping and sequence-within-group hopping. The latter two hopping approaches are jointly also referred to as Sequence/Group Hopping (SGH).
It has been found that the interference situation regarding reference signals strongly depends on the deployment and configuration of the communication systems as will now be described in more detail, again with exemplary reference to such communication systems that conform to the LTE standard.
Existing and upcoming realizations of LTE communication systems will include macro deployments, Heterogeneous Network (HetNet) deployments and Hotspot deployments. In a macro deployment, large cells are typically divided into independent sectors. In HetNet scenarios so-called pico cells are deployed within the coverage area of a macro cell (e.g, to improve coverage for high data rate applications). In a Hotspot implementation an access point serves a small coverage area with a high data throughput need.
LTE communication systems can also be designed with the aim of enabling optional Coordinated Multipoint Processing (CoMP) techniques, according to which different cell sectors or cells operate in a coordinated way (e.g., in terms of scheduling or processing). As an example, in the LTE uplink a signal originating from a single user terminal may be received by multiple receivers in different cell sectors or cells, and may then be jointly processed in order to improve the link quality. Uplink CoMP allows transformation of what traditionally is regarded as inter-cell interference into a useful signal.
One of the main innovations in the uplink for LTE Release 10 is the introduction of multi-antenna techniques for further increasing the data rate and communication reliability. The performance increase is highest in case both the transmitter and the receiver are equipped with multiple antennas (Multiple-Input Multiple-Output, or MIMO).
LTE Release 10 supports in the uplink Single-User MIMO (SU-MIMO). SU-MIMO is a spatial multiplexing mode in which a high rate signal is split into multiple lower rate data streams and each data stream (“layer”) is transmitted from a different transmit antenna in the same transmission bandwidth. Techniques such as linear precoding are employed by the user terminal to differentiate the layers in the spatial domain and allow a recovery of the transmitted data streams at the base station.
Another MIMO mode supported by LTE Release 10 is Multi-User MIMO (MU-MIMO) in which multiple user terminals belonging to the same cell are partly co-scheduled in the same transmission bandwidth and time slots. Each user terminal in the MU-MIMO mode may possibly transmit multiple layers, thus additionally operating in the SU-MIMO mode.
In case of SU-MIMO it is necessary to allow the receiver to estimate the equivalent channel associated with each transmitted layer (of possibly each user terminal) to allow detection of all data streams. In a CoMP scenario this requirement also applies to user terminals belonging to other cells but included in a joint processing cluster. Therefore, each user terminal needs to transmit a unique reference signal sequence at least for each transmitted layer. The base station is aware of the assignments between layers and reference signal sequences and performs layer-based channel estimation based thereon. The resulting channel estimate is then employed in the coherent detection process.
In case of MU-MIMO the user terminals may be scheduled on fully or partially overlapping transmission bandwidths. Depending on the particular MU-MIMO configuration, different requirements for reference signal generation result. For MU-MIMO within a cell and in case of fully overlapping transmission bandwidths, the reference signals of different user terminals may be multiplexed by means of cyclic shifts and/or OCCs. SGH may additionally be employed without affecting the orthogonality. For MU-MIMO within a cell and partially overlapping transmission bandwidths, the reference signals of different user terminals can be multiplexed by OCCs only, and SGH cannot be enabled for any of the user terminals. In the case of MU-MIMO for user terminals belonging to different cells (e.g., in a CoMP scenario), the user terminals are typically assigned different base sequences, and orthogonality can generally not be achieved.
Assuming an exemplary HetNet deployment, the small cell radius of the pico cell and the geographic location within a macro cell implies the presence of potentially strong interference between user terminals belonging to those cells. On the other hand, cell densification, increasing number of receive antennas and optional CoMP processing emphasize the need for flexible MU-MIMO scheduling.
In the scenarios described above it is generally not desirable to disable SGH and enhance the risk of inter-cell interference peaks. On the other hand, MU-MIMO is in most cases not efficient in conjunction with SGH if the user terminals within a specific MU-MIMO group are assigned different base sequences because neither cyclic shifts nor OCCs are effective in such a case, and thus only semi-orthogonality can be obtained.
One solution could be to assign the same base sequence (and consequently the same SGH pattern) to interfering cells such as macro cells and micro cells within macro cell coverage. However, this solution has its drawbacks, such as reduced SGH randomization, unpredictable large interference peaks generated when user terminals using the same base sequence are scheduled on partly overlapping transmission bandwidths, and reference signal capacity limitations. The capacity limitations result from the fact that only cyclic shifts and OCCs may be used for reference signal orthogonalization over the aggregated cells.
An alternative solution would be to disable SGH in a user terminal-specific manner as enabled in LTE Release 10. However, this solution has its drawbacks in case user terminals of LTE Release 8 or 9, for which SGH can only be enabled (and disabled) on a cell-basis, co-exist with user terminals conforming to LTE Release 10 in the same communication system. The co-existance implies that SGH can only be disabled in a cell-specific way also for user terminals conforming to LTE Release 10, which may result in a severe degradation of inter-cell interference.